Giardiasis, amoebiasis and cryptosporidiosis are the most common protozoal parasitic diseases of the human intestinal tract and common causes of enteric disease in non-human animal species as well. Different drugs have been used for the management of these diseases, but many have a significant failure rate in clearing parasites from the intestine and many are associated with a high incidence of undesirable side effects, which often leads to use being contraindicated in certain circumstances. There is a need in the art for an improved treatment which would require one or very few dosings, would be associated with a very high level of clearance of the target protozoal agent, would kill intestinal cysts (to reduce environmental contamination and the reservoir of reinfection), be associated with no or only mild and well tolerated adverse effects, have no pre-existing resistance, be safe for use in pregnant and lactating humans and other species and/or be suitable for use as a preventive medication.
Many existing antiprotozoal agents, especially those for the treatment of giardiasis, have suboptimal intrinsic efficacy. For metronidazole Giardia curative rates may be high, but have also been reported as low as 60% (measured by clearance of the protozoan) in human adult and paediatric patients when it is administered for 5-10 days. Both metronidazole and tinidazole have been reported to have a median efficacy of approximately 89%. The newer antigiardial agent, nitazoxanide has demonstrated an overall response rate of 75% (measured by clearance of the protozoan), ranging between 64 and 94%.
To improve compliance it is highly desirable that the dosage regimen is simple and of short duration. As outlined above, the duration of dosing with metronidazole is most commonly 5-10 days, leaving many opportunities for missed treatments, poor compliance and reduced efficacy or treatment failure.
Acquired resistance by Giardia to the available treatments is widely experienced and an increasing cause of treatment failure. This is not surprising as the majority of currently recommended antiprotozoal agents have been in use for many decades. A review of antiprotozoal agents approved by the FDA between 1987 and 2013 (Kesselheim, A. S. and J. J. Darrow (2014). “Drug Development and FDA Approval, 1938-2013.” New England Journal of Medicine 370(26): e39) discloses only 7 new agents for all protozoan diseases (mefloquine in 1989, eflornithine in 1990, halofantrine and atovaquone in 1992, nitazoxanide in 2002, tinidazole in 2004 and the combination of artemether and lumefantrine in 2009). Amongst these agents, only nitazoxanide and tinidazole are used for Giardia treatment, and both have median efficacies less than 90%.
Frequently it is the profile of adverse effects that limits the use of antiprotozoal agents. There is a diversity of adverse effects of currently available antiprotozoal agents. For example, side effects for the widely used metronidazole include metallic taste, nausea, headache, vertigo, leukopenia, insomnia and irritability. Less frequently, CNS toxicity has been reported especially with high doses, and alcohol consumption is not recommended when taking metronidazole because of the possibility of developing a reaction similar to that of disulfiram. Metronidazole has been shown to be mutagenic in bacteria; based on animal studies, the drug has been carcinogenic at high doses and over long periods. For quinacrine adverse effects include headache, nausea, vomiting and a bitter taste, resulting in poor compliance. Yellow discolouration of the skin, urine and sclerae may also follow its administration. Blue or black pigmentation of the nails, urticaria and exfoliative dermatitis can also occur. Other side effects reported are haemolysis in patients with glucose-6-phosphate dehydrogenase (G6PD)-deficiency, toxic psychosis and exacerbation of psoriasis. Quinacrine, in common with other antiprotozoal agents, is able to cross the placenta and reach the foetus, contraindicating its use during pregnancy due to a possible link with birth deformities. Other serious side effects described have included psychiatric disturbances.
For the treatment of cryptosporidia and amoebiasis caused by Entamoeba histolytica the limitations on available treatments are even more limited than with Giardia, with no consistently effective and specific treatment for cryptosporidia (Cabada, M. M. and A. C. White, Jr. (2010). “Treatment of cryptosporidiosis: do we know what we think we know?” Curr Opin Infect Dis 23(5): 494-499).
There is an unmet clinical need for antiprotozoan agents with novel mechanisms of action to supplement and/or replace currently available antiprotozoan agents, the efficacy of which is increasingly undermined by antiprotozoan resistance mechanisms. There additionally remains a need for alternative antiprotozoan agents in the treatment of infection by multi-resistant protozoa. However, as reported by the Pharmaceutical Research and Manufacturers of America in their 2013 report “Medicines in Development for Infectious Diseases” no novel antiprotozoan agents and few line extensions are being developed that offer promising results over existing treatments.
It is an object of the present invention to overcome at least one of the failings of the prior art.
The discussion of the background art set out above is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.